Automatic server maintenance. Being in calif...it starts at almost 1am, hence 3am-ish where they are. Sometimes it gets frustrating since I am a night owl, but thank goodness the net is vast and I can entertain myself until the maintenance is over. But by then, most everyone is in bed anyway.
Some platforms like this one have a store mode for saving YOUR work. Should say a server bot attack disable the server then once fixed and clean could re-load all work up to the 24 hour time frame it was attacked.